One problem facing the construction industry is controlling the quality or strength of concrete in structures such as buildings and roadways. Fresh concrete requires about twenty eight days to cure to its full strength, and so compressive strength tests can be performed on fully set concrete only after a significant delay between pouring of the fresh concrete and curing. Accelerated curing techniques have been employed to shorten the time, but any method that determines the strength of concrete after it has been poured into a structure at a construction site generally is considered too late.
A correlation between the water to cement ratio of fresh, unhardened concrete and the ultimate strength of a fully cured concrete has been used to predict the ultimate attainable strength of fresh concrete. In connection therewith, several methods have been proposed and there are some ASTM standards for separately measuring the water and cement contents of fresh concrete mix. Most of these methods generally rely upon laboratory procedures such as mechanical separation and weighing of the concrete components, which are time consuming and involved and are not suitable for fast, on-site determination of concrete quality. Examples of these methods are contained in U.S. Pat. Nos. 4,196,614 and 4,615,215.
Various nuclear apparatus and methods have also been proposed for separately determining either the water content or the cement content of a fresh concrete mix, but problems have been encountered. U.S. Pat. No. 3,911,271 to Mitchell relates to one such portable nuclear device for determining the cement content of fresh concrete, but the device apparently never was commercialized. The Mitchell nuclear cement content gauge used a sodium iodide scintillation crystal and was subject to electronic drift over time and in response to temperature changes. The Mitchell nuclear cement content gauge has been extensively reported on by the Federal Highway Administration in its report Nos. FHWA-RD-73-48, FHWA-RD-75-63, FHWA-RD-75-525, and FHWA-TS-78-201. See also report No. 284 of the National Cooperative Highway Research Program, which was published in June, 1986.
With the foregoing problems in mind, it is an object of the present invention to overcome the limitations of the prior apparatus and methods discussed above and to provide an apparatus and method for quickly and easily determining the physical properties of materials. A more specific object of the invention is to provide an apparatus and method that is suitable for field use in determining the moisture and cement content of fresh, unhardened concrete. A still more specific object of the invention is to provide an apparatus and method that is suitable for field use in determining the ultimate attainable strength of a fresh concrete mix.